This disclosure relates to micro-electromechanical system (MEMS) devices, in general, and in particular, to impulse-actuated MEMS shutters for miniature cameras.
In the familiar arcade game of “pinball,” momentum transfer is used to launch the game ball into play. Typically, a plunger having a specific mass is pulled back against the bias of a spring, and then released, causing the plunger to accelerate forward rapidly until it comes into contact with a stationary ball. Alternatively, the plunger can be rammed forward against the ball rapidly, e.g., with the heel of the hand. In either case, when the plunger contacts the ball, the momentum, or kinetic energy, of the moving plunger is transferred to the ball, causing it to separate from the plunger and accelerate rapidly forward in a desired trajectory. This basic principle can be used advantageously to actuate high-speed, miniature MEMS camera shutter mechanisms and the like through the use of highly efficient, high force, short-travel shutter blade actuators.
Miniature digital cameras are currently available with many electronic devices, such as cellular telephones, laptop computers, personal digital assistants (PDAs), and the like. Miniature cameras are also available as stand-alone devices for applications such as security and surveillance. Consequently, the market for such cameras is rapidly expanding. For example, camera-equipped cell phones now comprise a significant portion of the cell phone market. However, available miniature cameras may not be optimal for some applications.
Camera shutters control the transmission of light from a subject that passes through the camera's optical system to impinge upon a photo-sensitive material, e.g., a film containing silver iodide. In a conventional film (i.e., non-digital) camera, the shutter is positioned along the axis of the camera optics prior to film exposure, and prevents light from passing through the optics to the film. To initiate the exposure process, the user presses a shutter button, and responsively, the shutter moves to a position that allows light to pass through the camera optics to the film, and film exposure begins. After the desired exposure time has elapsed, the shutter moves back to the initial position so as to obscure the passage of light through the camera optics. Film advance mechanisms then move the exposed film away from the exposure position, while unexposed film is moved to the exposure position to be exposed at a later time.
Unlike film cameras, digital cameras need not include a mechanical shutter. Instead, shuttering may be performed electronically. However, some digital camera systems use a mechanical shutter in addition to electronic shuttering for, e.g., ensuring that the entire image is captured simultaneously and does not suffer from movement distortion. FIG. 1 shows an example of such a mechanical shutter mechanism 100 for a digital camera, according to the prior art.
The shutter mechanism 100 includes a mechanical shutter blade 130 with a pivot pin 135 and an actuator 140. The shutter mechanism 100 is included as part of a digital camera 110, which has a light aperture 120 configured to receive light to be processed to generate image information for an exposure. It should be noted that the term “exposure” in the context of digital photography refers to the time during which light is received for the photoelectric formation of a particular digital image, rather than a time during which film is exposed photo-chemically.
The digital camera 110 includes a controller (not illustrated) and a digital imaging system (not illustrated), such as a Complementary Metal-Oxide Semiconductor (CMOS) system or a Charge Coupled Device (CCD) imaging system, in place of conventional film. Received light corresponding to a matrix of image pixels is processed to generate a digital image, which is then stored in a memory device, such as an EEPROM.
In contrast to the conventional film camera described above, the shutter 130 of the digital camera 110 is initially positioned away from the aperture 120. The user pushes a button 125 to begin the exposure. In response, the controller resets the pixels of the digital imaging system to begin digital image data acquisition. At the end of the exposure time, the shutter 130 is then moved in front of the aperture 120 so as to block the passage of light through it and thereby end image data acquisition. In order to move the shutter 130, a force is applied using an actuator 140 that rotates the shutter 130 about the pivot point created by the pivot pin 135. After a short time, the actuator force is reversed, so that the shutter 130 moves back to its initial position away from the aperture 120.
Thus, in order to block the light entering the camera 110 at the end of the exposure, the shutter mechanism 100 must move a shutter “blade” disposed at the end of the shutter 130 through a distance sufficient to completely obscure the light aperture 120. This distance is typically relatively large and on the order of the size of the aperture. Conventional shutters have an actuator that applies a force that is nearly constant throughout the travel of the blade, and therefore, actuators with relatively large travel, or “throw,” are used. However, actuation force is typically inversely proportional to the travel range, so that it is difficult to achieve an efficient actuator that can move the required distance. Prior art shutters therefore typically use electromagnetic actuators to achieve sufficient force and long “throw,” or travel. However, these actuators consume large amounts of power, are relatively large, and are not amenable to efficient, precision MEMS fabrication technologies.
Accordingly, a need exists for shutter mechanisms incorporating a short-throw, but high-force actuator that can be used to accelerate a miniature camera shutter blade through a relatively long throw, or distance at a relatively high speed, and that is also amenable to fabrication using efficient MEMS fabrication technologies.